1. Field of the Invention
The invention relates to transmitter arrangements for multi-channel operation, and more particularly to arrangements in which switch-over to a back-up transmitter is desirable.
2. Description of the Prior Art
To prevent direct revenue loss, and loss of subscriber or listener loyalty, most commercial radio and television transmitter arrangements include some provision for continued broadcasting even though a portion of the regular transmitter has failed. Sometimes an entire duplicate transmitter and modulator are provided. However, such arrangements are quite expensive, so a common arrangement includes a back-up transmitter of less than half the power of the main transmitter. Antenna connections and. low level signal inputs usually must be changed from one transmitter to the other, and the back-up transmitter may require a significant warm-up time to provide stable operation. These make automatic switch over difficult and cause an undesirably long service interruption.
According to the invention, a multi-channel transmitting arrangement includes two transmitters, each of which in normal operation transmits a block of signals for approximately half the total bandwidth to a communications device such as a cable, waveguide or antenna; when the channels are similar, each transmits half the channels. In the event of failure of one of the transmitters, the signals for the block or channels it has been transmitting are provided to the other transmitter, which then transmits all the channels. Preferably the power transmitted per channel will remain substantially unchanged, although it may not be possible or practical to avoid an increase in distortion. Alternatively, the power level per channel may be reduced.
In normal operation, one block of channels may occupy the lower half of a band, and the other block occupies the upper half. An alternative arrangement may use two blocks with interleaved channels, where each block is spread over nearly the entire same band.
Because high level switching of antenna connections is not required, an arrangement according to the invention is relatively easy to automate, and no warm-up time is lost.
Whenever, in normal operation, each transmitter is operated below its maximum power output, for example to improve operating life or to reduce distortion, it is usually practicable that the back-up mode provide for transmission at the same power level per channel, so that service area coverage is not reduced. This is particularly applicable when adaptive power control is used to vary the transmitting power to accommodate time-varying changes in signal propagation over the service area, and the transmitters may be expected to be operating normally well below saturation.
In situations where signal attenuation in the service area fluctuates widely over the course of a year, transmitter operating cost considerations may make it desirable to operate the system such that, when minimum transmitter power will suffice, it is possible to turn off or to put one transmitter in a standby mode, and to use the switch to transfer all channels to the other transmitter. Where this mode is otherwise practicable, an important consideration will be the time required to activate, warm up or stabilize the transmitter which is not being actively used, so that high reliability of service can be maintained.
In a preferred embodiment, the transmitting arrangement handles a plurality of television or similar wide-band channels at frequencies above 10 to 12 GHz, such as a 1 GHz band between 27.5 and 28.5 GHz. For output powers of 120 watts per block, one traveling wave tube for each block of 25 FM channels provides sufficient power per channel to provide omnidirectional coverage out to a 3 mile radius. In normal operation, the lower frequency tube transmits channels 1-25 at 27.5 to 28.0 GHz, and the upper tube transmits channels 26-50 at 28.0 to 28.5 GHz.
In another embodiment, each transmitter includes a plurality of solid state amplifying or mixing/amplifying devices connected to one or a like plurality of antenna elements. Where each such device normally provides power for one small frequency block or respective channel of a large number of channels being transmitted, under standby operation each device would then transmit two such blocks or channels.
At some sacrifice in added complexity, the same inventive principle can be applied in situations where it is desired to have more than two amplifiers handling respective portions of the spectrum, and in the event of failure or poor performance by one, this portion can be shared among the others, or can be added in its entirety to the block transmitted by another.